Question

What happens when hydrogen gas is added to copper oxide?

Answer 1

Copper(II) oxide, CuO, will be converted to metallic copper when it is heated with hydrogen. This reaction is a typical redox reaction (oxidation-reduction)

Two instances of copper oxide reaction with hydrogen in real life applications are noteworthy. Both of these hydrogen reaction scenarios with copper oxide occur at elevated quickly at temperatures, 425 C. and above, although reduction is reported as low as 170C. (1)

1. When the copper oxide is on the surface of the metal (as in scale), the hydrogen reduces the copper oxide(s) to copper metal (Cu), the hydrogen forming H2O with reduction of the copper oxide.
   1. If the oxide layer is very thick (~0.25mm) on the copper base metal, such as scale, then the hydrogen often only reduces copper oxide at the top surface of the oxide scale leaving a very thin layer of pure copper on the un-reduced oxide beneath, This layer will peel off in a thin foil with subsequent operations.
2. When the copper oxide is present as cuprous oxide as an alloy component in copper matrix, hydrogen will diffuse easily through the copper matrix and reduce the Cu2O (cuprous oxide) in situ, also producing H2O which is now trapped in the copper matrix, typically at grain boundaries. The presence of the trapped H2O in the copper matrix results in an extremely brittle matrix. This deleterious condition is known in the copper industry as hydrogen embrittlement, and is unrecoverable.